Many modern medical procedures, both invasive and non-invasive, are performed with the assistance of medical imaging devices. A series of two-dimensional images generated from devices such as CT or MRI scanners are used to generate virtual 3-dimensional models of regions of interest from subjects. These models are used in the planning, practice and execution of medical procedures subsequently performed on the subjects. Image-guided medical procedures are generally accomplished by coordinating 3-D virtual models of subjects with reference points on the actual subjects undergoing the medical procedures. Current approaches for image-guided medical procedures such as stereotactic biopsy or radiation therapy can be divided into two approaches. The first approach uses a rigid frame secured to the subject, such as a human patient. The second approach is a frameless approach, in which fiducial landmarks, either derived from the patient's own anatomical features or applied to the surface of the patient, are used to coordinate the 3-D virtual patient model to the real world patient.
As shown in FIG. 1, in the rigid frame approach, a stereotactic frame is rigidly attached to the patient by placing fasteners directly to the skull. A detailed 3-D image map is then created from CT, MRI, or other 3-D imaging source. Fiducial markers placed on the stereotactic frame (not shown) appear in the images and allow objects in the image to be related to the stereotactic frame. Hence, targets of interest, such as a tumor and its surrounding anatomy, can be described using the coordinates of the stereotactic frame as a reference. The patient is then transferred to the operating room for treatment, again using the stereotactic frame as a reference. A biopsy arc is attached to the frame. A target trajectory is planned, and the arc parameters required to follow the trajectory are derived. The settings are applied to the biopsy arc and the biopsy is obtained. Although this approach eliminates the error-generating step of registering the virtual model to the actual subject, this approach is invasive and is as a result uncomfortable for patients.
In the frameless approach, the registration of the patient to the image-based operative model requires the identification of fiducial markers. Some fiducials are external markers applied to the patient prior to scanning and kept in place until registration has been completed, while other reference markers are actually identifiable anatomic landmarks based on the patient's own anatomy. The identification of these fiducial points can be difficult and can add significant time to the operative procedure. Additionally, movement of the fiducials relative to internal anatomy can degrade the accuracy of the registration process and subsequently detract from the overall accuracy of the image-guided procedure.
During the operative procedure with either frame-based or frameless approaches, tracking of the patient position as well as the position of the operative instruments is generally accomplished using one of two primary tracking technologies. The most popular system is optical tracking. Optical tracking systems depend upon a line of sight between the tracking camera and the tracked object, either the surgical instrument or a patient dynamic reference. Some optical systems track active infrared light emitting diodes while other systems track passive infrared reflective spheres. The second most popular system for patient and instrument tracking is electromagnetic tracking. In an electromagnetic tracking system, an emitter is typically used as the reference and is rigidly attached to the subject. All instruments are then tracked relative to the reference emitter. When electromagnetic tracking is employed, instruments that may distort the electromagnetic field, as well as other large pieces of electronic equipment, must be kept at sufficient distance from the surgical field to avoid significant spatial error introduction.
Both types of tracking systems suffer from drawbacks. In a surgical suite with multiple surgeons, support staff and support equipment, obtaining a “clean” electromagnetic environment needed for accurate electromagnetic tracking can be difficult. In the case of optical tracking, maintaining an unobstructed view of the operative field is often problematic and can lead to inaccuracies or the inability to track.